How to Use a Gray Card with a Digital Camera

There are two basic methods for setting the neutral balance with a gray card; prior to the moment of image capture or later in processing the image.

In the Camera Balancing

To apply a neutral balance at the time of image capture requires a camera that allows for setting a custom neutral balance (often called "white balance"). To make this balance a gray card is held in front of the lens and either an image is taken of the card or a button is pressed that causes the camera to measure the card and set the balance.

Position the card so the light hits the card at an approximate 45 degree angle. This is to minimize the specular reflection from the surface yet still have relatively even illumination across the card. The camera is aimed perpendicularly to the card surface.

Cameras use one of three types of gray balancing, depending on how much area the gray card must occupy in the image.

• Full frame - the gray must fill the entire field of view
• Focus area - the focus area must be filled with the gray
• Spot - a gray must be in the image

For the full frame and focus area cameras the gray card should be out of focus when making the neutral balance. This will hide any surface imperfections such as scratches, dust or texture, resulting in a better balance.

Full frame neutral balancing presents problems when using some lenses because the size of the gray card may need to be quite huge if an extreme wide-angle lens is used. If your camera needs this type of balancing and it has a zoom lens, setting the camera to its maximum telephoto setting will make it easy to use a smaller gray card.

Image Processing Balancing

This type of balancing requires an image with a gray card in it. In the image processing software a neutral balance tool is selected and clicked on the gray card.

Any camera can use image processing balancing since it is performed after the image capture. However, this may result in some banding in the processed image, depending on the shooting mode. Eight bit images are the most susceptible to this problem.

In the camera neutral balancing is the best choice for high quality images, followed by image processing balancing only if there is still some color cast in the image.

Camera Types

The following table shows some examples of the types of in-camera neutral balancing used in various cameras. This list is not meant to be exhaustive, but to show how important it is to check with the camera's manual for the details of custom neutral balancing for each model.

Image Processing Balancing

This type of balancing requires an image with a gray card in it. In the image processing software a neutral balance tool is selected and clicked on the gray card.

Note: a better neutral balance is achieved when the sample size in the image processing software is set to larger values. For instance, in Photoshop better sampling occurs when the eyedropper tool is set to a 5x5 spot area. This will average 25 pixels together for the result, improving the average and reducing the influence of noise in the sample.

Updated 19.9.2007

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